Much technology has been developed to facilitate communication of images over media of finite bandwidth. It is generally desirable to communicate the highest quality of images possible over a medium of a given bandwidth. Thus, techniques such as video compression (e.g., compression according to a Moving Picture Experts Group (MPEG) format) have been developed to reduce the amount of data required to represent images. An MPEG format includes various types of frames, including intra frames and non-intra frames. Intra frames contain sufficient information to reconstruct an uncompressed video frame without the need to reference information in other MPEG frames. Non-intra frames contain less information, allowing reconstruction of an uncompressed video frame when combined with information from other MPEG frames.
To increase the efficiency of the compression, the relationship between the intra frames and the non-intra frames varies depending on the nature of the video stream being encoded. For example, if a video stream includes frames that differ very little from one to the next, non-intra frames containing little information can accurately represent uncompressed video frames. However, if, for example, the frames of the video stream differ substantially from one another, more information is needed to accurately convey the video stream. As an example, during a scene change when the video stream changes from portray one scene to a completely different scene, the image of the new scene generally bears no relationship to the image of the previous scene. Thus, an intra frame is usually used to provide information about the new scene.
As can be readily appreciated, the relationship between the size of the intra frames and the non-intra frames, and even the frequency of the intra frames relative to the non-intra frames, cannot easily be predicted. Added complication arises when the compressed frames are to be communicated over a medium of finite bandwidth. While circumstances such as a scene change may necessitate communication of more information, the available bandwidth does not expand to accommodate the additional information. The buffers used to store information from the compressed video stream during processing are of finite size. Thus, variations in a compressed video stream can lead to buffer overflow and underflow conditions, disrupting the reproduction of the video stream. To accommodate the finite bandwidth of the medium, it is desirable to produce a compressed video stream that occurs at a constant, or substantially constant, bit rate.
The visual quality of compressed video encoded by a constant-bit-rate finite-buffer-size video encoder depends substantially on the characteristics of the underlying rate-control technique. To operate efficiently, the rate-control technique makes assumptions regarding the compression properties of future frames (i.e., frames that have not yet been compressed). These assumptions can be based on analyzing the compression properties of future frames in advance. While this leads to high quality and stable operation, it also causes an increase in computational and storage demands that is not always economic. Also the overall system delay increases significantly because a frame can only be encoded after the future frames needed for encoding this frame have become available. Thus, it is desirable to avoid these disadvantages.
In addition to the accurate prediction of the compression properties of future frames, it is desirable for a rate-control control algorithm to ensure that the number of actually generated bits for the current frame closely matches the target number of bits allocated to the current frame. Since the functional relationship between the primary control variable (e.g., the quantization step size) and the resulting number of bits is highly non-linear, iteratively encoding the frame at different quantization step sizes is used to exactly arrive at a given number of bits per frame. This is computationally expensive. Thus, it is desirable to avoid this computational expense and complexity.
Furthermore, it is desirable for rate-control to be robust. Whenever the assumptions, (e.g., the predicted compression properties of future frames or the number of bits generated for the current frame) turn out to be inaccurate, finite buffer-size constraints still have to be dealt with, preferably in a manner that does not greatly affect visual quality. Thus, it is desirable to provide such robustness so as to ensure that constraints are met and visual quality is maintained.
Thus, a technique is needed to provide rate control for a constant-bit-rate finite-buffer-size video encoder that provides the desired features while avoiding the disadvantages.